FIG. 1 shows an exemplary wireless telecommunications network 100. The illustrative telecommunications network includes base stations 101, 102 and 103, though in operation, a telecommunications network necessarily includes many more base stations. Each of base stations 101, 102 and 103 are operable over corresponding coverage areas 104, 105 and 106. Each base station's coverage area is further divided into cells. In the illustrated network, each base station's coverage area is divided into three cells. Handset or other user equipment (UE) 109 is shown in Cell A 108. Cell A 108 is within coverage area 104 of base station 101. Base station 101 transmits to and receives transmissions from UE 109. As UE 109 moves out of Cell A 108 and into Cell B 107, UE 109 may be handed over to base station 102. Because UE 109 is synchronized with base station 101, UE 109 can employ non-synchronized random access to initiate handover to base station 102.
A number of advanced features are being considered to enhance the cell throughput and/or the cell-edge throughput of the Evolved Universal Terrestrial Radio Access (E-UTRA) during the long term evolution (LTE) study stage. In coordinated multi-point reception (also know as macro-diversity), received signals from multiple cooperating units are combined and processed at a single location. This single location may be different for different user equipments (UEs). In different instances, cooperating units can be separate e-NodeBs, remote-radio units (RRUs), relays etc. The main concern is that coordinated multi-point reception requires coordinated multi-point synchronization.
Coordinated multi-point reception has a problem in different signal propagation delays from the UE to different cooperating units. FIG. 1 illustrates a first propagation delay τ1 between UE 201 and cooperating unit 211 and a second propagation delay τ2 between UE 201 and cooperating unit 212.
Proper receiver operation in any communication system requires appropriate timing synchronization with the transmitter. Multi-point reception requires the UE to be simultaneously synchronized to both cooperating unit receivers. In certain cases, this would be practically impossible to achieve. Whenever |τ1-τ2| exceeds the cyclic prefix duration this goal is practically impossible to achieve. However, it is possible to achieve simultaneous synchronization to different cooperating units in most other cases.
As in any other Orthogonal Frequency Division Multiplexing (OFDM) based system, the receiver timing can be regarded as a reference point for synchronization. All UEs talking to the receiver should adjust their transmission timing so that their signals arrive approximately simultaneously within the Cyclic Prefix (CP) tolerance at the receiver. Timing advance (TA) commands are sent to each UE to compensate for the propagation delay in the channel. Upon reception of a timing advance command, the UE adjusts its uplink transmission timing for Physical Uplink Control CHannel (PUCCH), Physical Uplink Shared CHannel (PUSCH) and sounding reference signals (SRS). The timing advance command indicates the change of uplink timing relative to the current uplink timing in multiples of a frame time constant. This propagation delay is commonly understood to be the first arriving path. For the example illustrated in FIG. 2, a TA command to advance timing, with respect to cooperating unit 211, by τ1, could be transmitted to the UE 201. Then the CP removal absorbs all the trailing paths from UE 201 to cooperating unit 211. In certain implementations the UE is expected to be heard by cooperating receivers at multiple different cooperating units, with different propagation delays. This situation is illustrated in FIG. 1 where UE 201 transmits to both cooperating receivers 211 and 211. Consequently, TA commands ought to depend on propagation delays to most or all of the cooperating units.